1. Field of the Invention
This invention relates to radiodiagnostic reagents and peptides, and methods for producing labeled radiodiagnostic agents. Specifically, the invention relates to peptides that can be labeled with technetium-99m (Tc-99m), methods and kits for making and radiolabeling such peptides, and methods for using such peptides to image sites of thrombus formation in a mammalian body.
2. Description of the Prior Art
Thrombosis and thromboembolism, in particular deep vein thrombosis (DVT) and pulmonary embolism (PE), are common clinical conditions that are associated with significant morbidity and mortality. It has been estimated that in the U.S. approximately 5 million patients experience one or more episodes of DVT per year and that over 500,000 cases of pulmonary embolism occur, resulting in 100,000 deaths (J. Seabold, Society of Nuclear Medicine Annual Meeting 1990). It has also been estimated that over 90% of all pulmonary emboli arise from DVT in the lower extremities. Anticoagulant therapy can effectively treat these conditions if applied early enough. However, such treatment is associated with risks (e.g. internal bleeding) that prevent unnecessary prophylactic application. More advanced techniques of thrombolytic intervention (such as the administration of recombinant tissue plasminogen activator or streptokinase) can be used in acute cases, but these techniques carry even greater risk. Moreover, effective clinical application of these techniques requires that the site of the offending thrombus be identified so as to monitor the effect of treatment.
For these reasons, a rapid means of localizing thrombi in vivo, most preferably using non-invasive methods, is highly desirable. Methods currently utilized for the identification of thrombolytic sites are contrast venography and compression B-mode ultrasound; the choice of which technique is used depends on the expected location of the thrombus. However the former technique is invasive and both techniques are uncomfortable for the patient. In addition, these methods are in many cases either unsuitable or yield inaccurate results.
In the field of nuclear medicine, certain pathological conditions are localized, or their extent is assessed, by detecting the distribution of small quantities of internally-administered radioactively labeled tracer compounds (called radiotracers or radiopharmaceuticals). Methods for detecting these radiopharmaceuticals are known generally as imaging or radioimaging methods.
In radioimaging, the radiolabel is a gamma-radiation emitting radionuclide and the radiotracer is located using a gamma-radiation detecting camera (this process is often referred to as gamma scinitigraphy). The imaged site is detectable because the radiotracer is chosen either to localize at a pathological site (termed positive contrast) or, alternatively, the radiotracer is chosen specifically not to localize at such pathological sites (termed negative contrast).
A number of factors must be considered for optimal radioimaging in humans. To maximize the efficiency of detection, a radionuclide that emits gamma energy in the 100 to 200 keV range is preferred. To minimize the absorbed radiation dose to the patient, the physical half-life of the radionuclide should be as short as the imaging procedure will allow. To allow for examinations to be performed on any day and at any time of the day, it is advantageous to have a source of the radionuclide always available at the clinical site.
A variety of radionuclides are known to be useful for radioimaging, including .sup.67 Ga, .sup.99m Tc (Tc-99m), .sup.111 In, .sup.123 I, .sup.125 I, .sup.169 Yb or .sup.186 Re. Tc-99m is a preferred radionuclide because it emits gamma radiation at 140 keV, it has a physical half-life of 6 hours, and it is readily available on-site using a molybdenum-99/technetium-99m generator.
A gamma-emitting radiotracer that binds specifically to a component of a thrombus in preference to other tissue when administered in vivo can provide an external scinitigraphy image which defines the location of the thrombus-bound radiotracer and hence the thrombus. There are several potential radiotracer targets in thrombi. Thrombi are constructs of blood cells, largely platelets, enmeshed in cross-linked fibrin. Venous thrombi are fibrin-rich, whereas arterial thrombi are platelet-rich. Fibrin and platelets are thus obvious targets for designing radiopharmaceuticals for imaging thrombi, each having multiple possible target sites.
Activated platelets and fibrin have been used as targets in radioimaging thrombi because neither are normally found in circulating blood; circulating blood contains unactivated platelets and fibrinogen, a fibrin precursor. Thrombus formation involves the proteolytic conversion of fibrinogen to fibrin and the physiological conversion of unactivated platelets to an activated state. Since little fibrin circulates in the blood stream (in contrast to its precursor, fibrinogen) and since most circulating platelets are unactivated, fibrin and activated platelets are excellent and specific targets for imaging thrombi because they will not be found to any substantial extent anywhere in vivo other than in the thrombus.
The use of radiolabeled fibrinogen and radiolabeled platelets for radioimaging has a number of disadvantages, however. Blood and background tissue clearance of radiolabeled fibrinogen and platelets are slow, which necessitates a long delay between injection and imaging. Also, as thrombi age radiolabeled platelets become less efficient imaging agents, although fibrin and platelets already in an existing thrombus remain targets even in aged thrombi.
Attempts to provide radiotracers for imaging thrombi are known in the prior art. These include autologous platelets, labeled with either .sup.111 In or .sup.99m Tc (Tc-99m), and .sup.123 I- and .sup.125 I-labeled fibrinogen (the latter detected with a gamma scintillation probe as opposed to a gamma camera). In addition, other thrombus-associated components of the coagulation system, such as enzymes (e.g. thrombin), proenzymes and other factors may be useful as thrombus-associated targets for radiotracers. Additional radiolabeled compounds used to label thrombi include plasmin, plasminogen activators, heparin, fibronectin, fibrin Fragment E.sub.1 and anti-fibrin and anti-platelet monoclonal antibodies [see Knight, 1990, Sem. Nucl. Med. 20: 52-67 for review].
Of the methods of radiolabeling thrombi known in the prior art, the methods that have shown the most promise are radiolabeled platelets, radiolabeled antibodies and radiolabeled fibrin Fragment E.sub.1. All of these have serious drawbacks with regard to their routing use.
The use of radiolabeled autologous platelets to image thrombi requires that autologous blood be drawn, the platelets then separated and radiolabeled under sterile conditions [in addition, radiolabeling must be performed so as to avoid activating the platelets], and in radiolabeled platelets then be readministered to the patient. Such radiolabeled platelets have a long circulating time, resulting in poor target to non-target ratios at early times, thereby requiring that radioimaging be performed only after a delay of 24 to 72 hours. Moreover, aged thrombi are poorly visualized since such thrombi do not efficiently incorporate fresh platelets.
Radiolabeled antifibrin and antiplatelet monoclonal antibodies have also been used in the prior art (typically to image DVT). The disadvantage to using such reagents is that antibodies (and even antibody fragments) have slow blood and general tissue clearance characteristics and require a delay of at least several hours for optimum imaging. In addition, immunological reagents have the capacity to induce an immune response in the patient. Further, such reagents must be prepared from mammalian cell lines (hybridomas) and thus carry the risk of contamination by infectious human viruses.
Methods for using large radiolabeled peptides for imaging thrombi have been described in the prior art. For example, Fragment E.sub.1 is a peptide derived from coagulated, cross-linked fibrin. It has bee labeled with .sup.123 I and Tc-99m to provide high quality images in humans.
Olexa et al., 1982, European Patent Application No. 823017009 disclose a pharmaceutically acceptable radiolabeled peptide selected from Fragment E.sub.1 isolated from cross-linked fibrin. Fragment E.sub.2 isolated from cross-linked fibrin, and peptides having an amino acid sequence intermediate between Fragments E.sub.1 and E.sub.2. Unfortunately, this protein fragment must be laboriously prepared from human fibrinogen, making it unsuitable for routine manufacture.
Hadley et al., 1988, PCT/US88/03318 disclose a method for detecting a fibrin-platelet clot in vivo comprising the steps of (a) administering to a patient a labeled attenuated thrombolytic protein, wherein the label is selectively attached to a portion of the thrombolytic protein other than the fibrin binding domain; and (b) detecting the pattern of distribution of the labeled thrombolytic protein in the patient.
Sobel, 1989, PCT/US89/02656 discloses a method to locate the position of one or more thrombi in an animal using radiolabeled, enzymatically inactive tissue plasminogen activator.
Peptides having the ability to bind to thrombi are known in the prior art.
Ruoslahti & Pierschbacher, U.S. Pat. No. 4,578,079 describe peptides of sequence X-Arg-Gly-Asp-R-Y, where X and Y are either H or an amino acid, and R is Thr or Cys, the peptides capable of binding to thrombi in vivo.
Ruoslahti & Pierschbacher, U.S. Pat. No. 4,792,525 describe peptides of sequence Arg-Gly-Asp-X, wherein X is Ser, Thr or Cys, the peptides capable of binding to thrombi in vivo.
Hawiger et al., 1989, PCT/US89/01742 relates to peptides comprising sequences for two binding sites of a protein.
Barker et al., 1991, PCT/US90/03788 disclose cyclic peptides for inhibiting platelet aggregation.
Radiolabeled peptides useful for radioimaging thrombi have been reported in the prior art.
Ranby et al., 1988, PCT/US88/02276 disclose a method for detecting fibrin deposits in an animal comprising covalently binding a radiolabeled compound to fibrin.
Stuttle, 1990, PCT/GB90/00933 discloses radioactively labeled peptides containing from 3 to 10 amino acids comprising the sequence arginine-glycine-aspartic acid (RGD), capable of binding to an RGD binding site in vivo.
Rodwell et al., 1991, PCT/US91/03116 disclose conjugates of "molecular recognition units" with "effector domains".
Maraganore et al., 1991, PCT/US90/04642 disclose a radiolabeled thrombus inhibitor comprising (a) a inhibitor moiety; (b) a linker moiety; and (c) and an "anion binding exosite (ABE)" binding site moiety.
The use of chelating agents for radiolabeling polypeptides, and methods for labeling peptides and polypeptides with Tc-99m are known in the prior art and are disclosed in copending U.S. patent application Ser. Nos. 07/653,012, 07/807,062, and 07/851,074 which are hereby incorporated by reference.
There remains a need for a small (to enhance blood and background tissue clearance), synthetic (to make routing manufacture practicable and to ease regulatory acceptance) molecules radiolabeled with Tc-99m for use in imaging thrombi in vivo. Small synthetic peptides radiolabeled with Tc-99m that bind specifically to components of thrombi fulfill this need and are provided by this invention.